1. Field of the Invention
The present invention relates to a telecommunications system for implementing telecommunications between a terminal other than a telephone, i.e., a non-telephone terminal accommodated in a switched telephone network or similar telephone switching network and a supervising center which is also accommodated in the telephone network and controls a plurality of non-telephone terminals.
2. Description of the Prior Art
The inspection of an LPG (Liquefied Petroleum gas) meter, for example, may be automated by providing a subscriber with a reading terminal which shares the same line as a telephone and allowing a supervising center which supervises the reading terminal to communicate with the terminal over a telephone switching network so as to collect data from the terminal, send alarm information to the terminal, and perform other various functions as needed, as already proposed in the telecommunications art. A predominant approach for implementing such telecommunications by a relatively simple configuration is a terminal origination type system.
In a prior art terminal call origination type system, a terminal adapter is connected to a telephone line or subscriber line which is accommodated in a switched telephone network, while a reading terminal and a subscriber set telephone are connected to the terminal adapter. The reading terminal is capable of accessing a supervising center over the telephone switching network by calling the telephone network via the terminal adapter. The terminal adapter plays the role of a network control unit which selectively connects the telephone and reading terminal to the telephone switching network.
A prerequisite with the automatic LPG supply service is that the reading terminal sends readings and similar data periodically to the supervising center while the center sends data to the reading terminal. By sending data to the reading terminal, the center may govern the reading terminal for various purposes such as checking the terminal when collected data is unusual, on-off controlling the gas valve in the event of the subscriber's long absence, examining the conditions of use of LPG as needed, resetting a counter installed in the terminal to zero, informing the terminal of a change of the telephone number of the center itself, and correcting a timepiece built in the terminal adapter. While some of these control items are sufficiently achievable even if a command in the form of data is transmitted by using a communication path which will be set up when the terminal accesses the supervising center, the others cannot be implemented unless the center delivers a command whenever occasion calls. However, since the terminal call origination type system does not allow the supervising center to call the terminal, the control items of the kind needing occasional delivery of command as mentioned above have to rely on some alternative means such as a conversation on a telephone or the dispatch of a person in charge to the terminal.
In the light of the above, there has been proposed by Nippon Telegraph and Telephone Co., Ltd, an "Interface for No-Ringing Communication Service" which allows a central station to call a terminal at any time, i.e. a center call origination type system. In the proposed center call origination type system, a no-ringing trunk is provided in a local switch in which a reading terminal is accommodated through a terminal adapter, i.e. a receiving local switch. Built in the local switch for calling a reading terminal, the no-ringing trunk has appearances which are individually accommodated in the incoming and outgoing ends of the local switch. When the central station originates a call meant for the reading terminal, a ringing having a particular frequency is sent from the no-ringing trunk to the terminal adapter. On receiving the ringing, the terminal adapter recognizes that call is destined to the associated reading terminal and thereby connects the telephone line to the terminal.
A drawback with the center call origination type system is that the special no-ringing trunk installed in the receiving local switch increases the cost of the entire switch. Especially, when the number of non-telephone terminals accommodated in the local switch is relatively small, the cost-effectiveness is critically lowered by the installation of the no-ringing trunk in the local switch. This obstructs the extensive application of the control of non-telephone terminals by a supervising center.